class Solution {
    int ret, step, m, n, endx, endy;
    bool visited[20][20];
    int dx[4] = { 0, 0, 1, -1 }, dy[4] = { 1, -1, 0, 0 };
    void dfs(vector<vector<int>>& grid, int i, int j, int count)
    {
        if((endx == i && endy == j) && (count == step))
            ret++;

        for(int k = 0; k < 4; k++)
        {
            int x = i + dx[k], y = j + dy[k];
            if(x >= 0 && x < m && y >= 0 && y < n && grid[x][y] != -1 && visited[x][y] == false)
            {
                visited[x][y] = true;
                dfs(grid, x, y, count + 1);
                visited[x][y] = false;
            }
        }
    }
public:
    int uniquePathsIII(vector<vector<int>>& grid) {
        m = grid.size(), n = grid[0].size();
        int startx = 0, starty = 0;
        for(int i = 0; i < m; i++)
        {
            for(int j = 0; j < n; j++)
            {
                if(grid[i][j] != -1)
                    step++;
                if(grid[i][j] == 1)
                    startx = i, starty = j;
                if(grid[i][j] == 2)
                    endx = i, endy = j;
            }
        }
        visited[startx][starty] = true;
        dfs(grid, startx, starty, 1);
        return ret;
    }
};